In recent years, the deterioration of many concrete bridge structures has become a major maintenance problem. Although the deterioration of concrete is influenced to some degree by the quality of the concrete, loading conditions and environmental exposure, the primary cause of deterioration is the result of corrosion of the reinforcing steel. The reinforcing steel embedded in concrete is passive unless the concrete becomes contaminated with chlorides from deicing salts or salt-water spray from coastal waters.
Due to its open-cell structure, concrete can allow critical quantities of chlorides to permeate its structure to the level of the embedded reinforcing steel. The chloride ions in the presence of moisture and oxygen initiate the corrosion of the reinforcing steel. As the corrosion products are formed, there is an increase in the volume of the steel, thereby exerting tensile forces on the surrounding concrete. As these forces become larger than the tensile strength of the concrete, the concrete cracks, thus allowing additional paths for the intrusion of water and chlorides. Eventually the concrete spalls and/or delaminates, and the concrete structure can become completely deteriorated.
The use of impressed current cathodic protection (CP) in chloride-contaminated concrete bridge decks is well known. In the early CP systems developed for bridge deck applications, an electrically conductive asphaltic-concrete overlay was used which performed well but was not useful for overhead and vertical structures. Alternative CP systems utilize electrically conductive polymer concrete mortar in slots cut into the bridge deck surface or electrically conductive polymer concrete mortar strips on top of the concrete deck with a protective concrete overlay over them. However, none of these CP systems were appropriate for application to any concrete surfaces other than bridge roadways.
Recently, it has become increasingly clear that the protection of the bridge substructure is as important as the deck. The protection of vertical and overhead concrete elements has been hampered by the lack of a suitable material and application method to effectively distribute the electrical current over the entire concrete surface. The inappropriateness of the prior art CP systems for use in protecting bridge substructure elements has been overcome with the formulations of the present invention. In one embodiment of the present invention, there is provided an electrically conductive polymer concrete coating suitable for spray application to vertical or overhead concrete surfaces. The coating uniformly distributes the cathodic protection currents across the concrete surface and is permeable to allow gases generated within the concrete substrate during the cathodic protection operation to pass through the coating.